Numerous attempts have been made in the past to produce a safe, reliable and inexpensive electrically energized flat heating tape useful for heating floors, walls and the like. The process of generating heat by passing a current through carbon or semi-conductive material is very old, and many attempts have been made in the past to introduce a tape of a simple design which is inexpensive to manufacture, and furthermore is free of the danger of overheating which could produce a fire hazard.
Electrically energized heating tapes should also be rugged and capable of being tightly rolled up in a compact manner for shipment and storage. One prior art approach is to utilize high resistance metallic conductors such as nichrome wires embedded within a plastic substrate, whereby the conductors are coupled in series to generate heat. When a pair of these wires are employed in such a substrate, they must be connected in series to form a closed circuit, after the substrate is cut to a desired often indefinite, length. However, providing the necessary connections between the two wires at terminal portions of the tape after being cut, is an annoyance, and the connecting device could be dangerous when 115 volts are employed to energize the tape. Also, the length of the tape has to be related to the applied voltage, and hence the user of the tape is not free to cut a desired length of tape.
U.S. Pat. No. 3,387,248 to Rees, teaches overlaying a carbon conductive substrate with a pair of conductive strips so that the carbon substrate bridges the conductors, rendering electrical connection between end portions of the twin parallel conductors unnecessary, in contrast with the aforesaid arrangement utilizing twin nichrome wire leads connected in series. However, electrically conductive adhesives are utilized to bond the parallel conductors to the graphite substrate, and the use of an additional adhesive creates problems, since the tape is often curled when applied to a pipe for example, or during roll-up upon shipment, in turn causing a loosening of the otherwise firm connection necessary to maintain uniform current flow at the junction between the parallel conductors and the resistive substrate. This problem is evidenced by the statement in column 2 of the patent, that "in order to improve the fixation of the electrode it may also be anchored mechanically to the sheet or layer, for example, by interlacing it with the weave of the fibrous support, when one is employed, or alternatively crimping the electrode to the layer or sheet prior to embedding in the cold setting rubber."
Any loosening of the junction between the twin electrical conductors and the heating substrate is very detrimental since "hot spots" may be produced due to the resulting high voltage gradient across the air gap between the surface of the electrical conductor and the material making up the resistive element. Such "hot spots" are a fire hazard, due to the resulting sparking within the air gaps producing overheating, and even possible destruction of the electrical connector junction point.
The aforesaid mechanical anchoring approach is similar to the approach of stitching or stapling the twin electrical conductors to the conducting heat generating substrate as taught by U.S. Pat. No. 3,385,959 to Ames. This approach is by it's very nature crude and a nuisance to implement in the manufacturing process. Additionally, the desired flexibility in the tape, is generally not permitted through the use of these techniques; the manufacturer should be able to ship the heating tape in relatively compact rolls, while the tape should be capable of being bent about sharp corners during installation.
In U.K. Pat. No. 2,065,430, a pair of conductive strips are positioned over a carbon heating substrate. In this patent there is no suggestion of a bonding agent between the twin conductors and the substrate, and thus the tape will only function without "hot spots" if it is wrapped around a tube or pipe to maintain the conductors tightly against the heating substrate, and this application is emphasized in FIG. 1 of the patent.
I am quite familiar with the U.S. Pat. No. 4,485,297 to Grise et al. since I personally designed some of the manufactured components for the inventors. Heating tapes are presently manufactured in accordance with the teachings of this patent, employing a striped pattern of granular carbon which is silk screened upon the substrate. This method is costly, and requires a closely controlled thickness of the carbon paste mixture making up the stripes and the printed width of each heating strip to prevent the formation of air gaps and the resulting detrimental hot spots. The carbon strips have to be of high conductivity, to create a low enough resistance to generate sufficient heat. The hot spot problem is approached by increasing the thickness of the carbon stripes at the contacts, such contacts having a curved configuration as illustrated in FIG. 2 of the patent. This creates a kind of "sandpile" under the curved conductors so that when the tape is rolled or flexed, the particles tend to roll under the curved conductive strip in order to maintain contact, and hence minimize the formation of the air gaps leading to sparking and hot spots.
It is thus highly desirable to create a simple design of a heating tape which is inexpensive to manufacture and produces consistent quantities of heat upon the application of a given voltage to the tape without the danger of overheating due to hot spots created by non-uniformity in the electrical junctions themselves, or non-uniformity of the resistive material generating the heat. It is also highly desirable to provide an ultra thin tape which tends to minimize the above mentioned problems, which may be readily rolled up without stretching the electrical connectors between the resistive layer and the supply conductors, resulting in uneven current flow and possible hot spots, and which does not employ a failure-prone electrically conductive adhesive between the supply conductors and the resistive bridging element.